Compressed air won't fix it the fan is open circuit. Ordered a replacement from China but despite waiting two weeks the new one arrived and it too was faulty. I can only hope the replacements replacement is OK.

If it is a later-model Sony DVD/HDD unit (x50, x70, x80, x90 series) made in cooperation with Pioneer, the fan will have two primary speeds: relatively slow and quiet when the HDD is selected, and faster/louder when DVD is selected. The ramp up in fan speed when switched to DVD mode is immediate and striking: if you're standing anywhere near the recorder while playing or recording a dvd, you'll hear a distinctly audible whine. This will gradually fade if you switch back to HDD: within five minutes the fan should return to quiet mode.

So if you don't hear a noticeable fan whine in DVD mode, the fan is defective or the board circuit controlling it has gone bad.

Thank you Orsetto, that makes more sense as the fan has three wires, brown +ve, black -ve, and grey. The new fan is identical except for wire colour and length, (shorter). The board terminals for the black and brown read no voltage but I'm reluctant to run it without cooling. I measure 5v on the other terminal. This is probably the slow speed voltage, so I'll have to be patient and wait another two weeks for China air post to supply the replacement.

I've been digging deeper and was wrong regarding the original fan being o/c. No reading on meter (200 ohms range) but connect a 9v battery and it runs. So fault is with board, damn. I may look at rigging up an external fan to get the data off. Don't wish to lose my grandmother and father doing a turn in 1934, and the launching of the Queen Mary.

Yeah, I'm a fan of that as well.
When fans die or start to get loud, I always replace them by a silent pc fan with at least the same capacity. A lot of times there is simply enough room to build one in. If power to the fan has died I may wanna check the power to it's controller board is still ok and try if I can get direct power from that.

In an amplifier I once soldered a simple small powersupply to the mains for a bigger, more silent replacement fan I build in.

All very helpful, thanks. I'll test for a reliable supply and probably add the replacement fan. There maybe a problem disconnecting the original as I think it needs to see the fan to operate. So plenty of room to move it aside and fit the new one. Just need to test all the outputs from the nearby PSU.

I measure 5v on the other terminal. This is probably the slow speed voltage...

Possible, sometimes 2 voltages are used for different speeds. There is also the possibility that the fan operation voltage is 5 volts. You can check on the fan; often this information is on the fan itself. Or you can google the typenumber for specs.

Should it be a 5V fan, than it's possible the third wire is there for the tacho-signal, it "tells" the controllerboard how fast it runs. The controllerboard can then adjust the voltage (with a 5V max) and so regulate the speed. I believe a lot of 3-wire pc fans work the same.

Should the board be dead, I'd just put in a fan with constant power supply and don't worry that it'll run constantly. Your hardware won't freeze to death...

Update: The 5v seems to be the sensing wire as mentioned above, and the fans rating is 12v. It runs well on a 9v battery, so that's probably the idle speed, switching to 12v for extra cooling when burning discs (them lasers get hot). All supplies are good from PSU, so added a wire from 12v, I'll add a switch and a wire from 9v, so I can switch speeds when needed.

I'm not familiar with these machines but I would caution you that fan speed control isn't always just a case of increasing or decreasing the voltage. Most fans these days use an electronic driver circuit with a magnetic sensor (Hall device) in them so measuring resistance across the wires can't be used as a method of testing. Some also use constant voltage but PWM drive to adjust the speed. Basically, full voltage is pulsed on and off rapidly (possibly several tens of thousands of times every second) but the ratio of 'on' to 'off' times are changed to adjust the average power and hence speed. It has become common practice to use PWM in newer designs because it is more efficient and can be controlled in software but it also makes it impossible to measure reliably using a conventional test meter.

Also, bear in mind that if the speed is controlled by PWM, there must be a sensor somewhere to measure the temperature and it may have a lower threshold to start the fan running, for example it may not start at all until say 40C is reached so an apparently dead fan may intentionally not be powered up.

Thanks for all the interest guys, the initial fault symptom was that the unit would power up then shutdown which was due to over temp. So the fan not running till a predetermined temp is reached isn't right. I have run the unit over night with the fan connected to UN12V and ground. Switch has been fitted, just need to test on UN6V and UN4V.